1. Field of the Invention
The invention in general pertains to the production of dialkyl or diaryl phosphorochloridothionates and, in particular, to a method for treating the residue from phosphorochloridothionate production and the recovery of additional product therefrom.
2. Prior Art
It has been suggested in the prior art to use the distillation residue from dialkyl phosphorochloridothionate production to produce additional thionate product (see British Patent No. 1,245,052; U.S. Pat. Nos. 4,025,586 and 4,078,023).
British Patent No. 1,245,052 discloses a method removing the sulfur from the residue and using the remaining residue as additional feed material for further product recovery. The sulfur is said to be in liquid form prior to separation from the residue mixture.
U.S. Pat. No. 4,078,023 discloses separating liquid sulfur from the residue after chlorination and distilling the residue prior to recycling the distillation portion of the residue for additional product recovery.
Recycling the residue as disclosed in the above cited art causes hydrolyzable impurities to accumulate, particularly the polymerized sulfur compounds which do not separate out with simple phase separation.
U.S. Pat. No. 4,025,586 addresses the problem of the buildup of hydrolyzable impurities by washing the residue with water, after separation from the solid elemental sulfur, to remove the hydrolyzable impurities and thereafter drying the residue prior to recycling it for chlorination. The disclosure specifies the use of known means for separating the solid sulfur from the residue and discloses as an example centrifugation.
In some processes for making dialkyl phosphorochloridothionates, there is obtained a reaction mixture comprising amorphous, generally polymeric sulfur, which is generally, but not necessarily, in solution in an inert liquid reaction medium of the dialkyl phosphorochloridothionate product. The product is recovered by distillation leaving a distillation residue comprised of sulfur, which depending on the purity of the initial reactants is either in a liquid or a plastic solid state; other unstable organophosphorus byproducts; amounts of bis(thiophosphono) sulfide or polysulfide; and small amounts of the dialkyl or diaryl phosphorochloridothionate product.
One method of producing phosphorochloridothionates is from a corresponding phosphorodithioic acid intermediate produced by reacting P.sub.2 S.sub.5 with an alcohol. It is generally desirous to use very pure reactants and maintain accurate reaction conditions to obtain high product yields and minimize the amount of side reaction products (impurities), formed in the process. The product is generally separated by fractional distillation leaving a distillation residue comprised of the liquid or solid sulfur and other unstable organophosphorus compounds. If the starting materials are pure and precise reaction conditions are maintained, the reaction products will generally contain reduced amounts of impurities and could possibly be immediately separated from the remaining distillation residue by phase separation. More generally however, the commercial process utilizes reactants which contain varying amounts of impurities, and reaction conditions may also vary somewhat. The distillation residue obtained under such conditions will generally comprise a mixture of solid plastic amorphous sulfur in a liquid comprised of organophosphorous bearing compounds and other dissolved polymeric sulfur compounds. Sulfur, especially solid amorphous sulfur is very difficult to separate from the other components of the distillation residue by conventional methods.
It is an object of the invention to provide means for separating the solid amorphous sulfur from the distillation residue where by the unreacted bis (thiophosphono) sulfide and the remaining dialkyl phosphorochloridothionate product can be recovered and recycled.